1. Field of the Invention
The present invention relates generally to methods and compositions for fracturing subterranean formations penetrated by a well bore. More particularly, the present invention relates to methods and compositions for fracturing formations wherein a fracturing fluid containing up to about 95 percent carbon dioxide by volume is injected into a formation, with or without a propping agent suspended therein, through a well bore at a rate sufficient to open a fracture in the formation.
2. Description of the Prior Art
The treatment of subterranean formations penetrated by a well bore to stimulate the production of hydrocarbons therefrom or the ability of the formation to accept injected fluids has long been known in the art. One of the most common methods of increasing productivity of a hydrocarbon-bearing formation is to subject the formation to a fracturing treatment. This treatment is effected by injecting a gas, liquid or two-phase fluid down the well bore at sufficient pressure and flow rate to fracture the subterranean formation. Continued pumping of the fracturing fluid containing a propping agent, such as for example, sand, fine gravel, sintered bauxite, glass beads or the like into the fracture results in placement of the proppant within the fracture. Following the treatment, the fracturing fluid is recovered from the well bore or permitted to migrate from the fracture leaving the propping agent remaining in the fracture. The propping agent prevents the complete closure of the fracture to provide a flow channel through which an increased quantity of a hydrocarbon or other fluid can flow.
The conductivity of the propped fracture depends, among other things, on the size of the propping agent particles placed in the fracture. This, in turn, depends upon the width to which the particular fracture may be opened during injection of the fracturing fluid and this normally requires that such fluids have high viscosities. The use of fracturing fluids having high viscosities is advantageous since such fluids can support the propping agent particles suspended therein without excessive settling.
A traditional fracturing technique utilizes a water or oil-based fluid containing a gelling agent to fracture a hydrocarbon-bearing formation. To facilitate recovery of the fracturing fluid from the well bore, various improvements have been made in the fracturing techniques. One such improved technique is that known as foam fracturing. This process is described in, for example, U.S. Pat. No. 3,980,136. Briefly, that process involved generation of a foam of a desired "Mitchell quality" which then is introduced through a well bore into a formation which is to be fractured. Various gases and liquids can be used to create the foam, but foams generally used in the art are made from nitrogen and water, in the presence of a suitable surfactant. The pressure at which the foam is pumped into the well is such that it will cause a fracture of the hydrocarbon-bearing formation. Additionally, the foam comes out of the well easily when the pressure is released from the well head, because the foam expands when the pressure is reduced.
Yet another fracturing technique has been that utilizing a liquefied, normally gaseous fluid. U.S. Pat. No. 3,195,634, for example, discloses a method for treating a subterranean formation penetrated by a well bore with a composition comprising a liquid-liquid mixture of carbon dioxide and water. The carbon dioxide is present in an amount equivalent to from about 300 to about 1500 SCF at 80.degree. F. and 14.7 psia per 42 gallons of water. The composition is injected into the formation under sufficient pressure to fracture the formation. The composition can include gelling agents and proppant materials. Upon pressure release at the well head, the liquid carbon dioxide vaporizes and flows from the formation.
U.S. Pat. No. 3,310,112 discloses a method of fracturing a subterranean formation penetrated by a well bore comprising introduction of a mixture of liquid carbon dioxide and a propping agent slurried in a suitable vehicle into the well bore at a pressure sufficient to fracture the formation. The liquid carbon dioxide is present in an amount sufficient to provide at least five volumes of carbon dioxide per volume of slurried propping agent. After injection of the liquid carbon dioxide containing the propping agent, the pressure on the well bore is released. The liquid carbon dioxide normally is heated sufficiently by the formation that upon pressure release, the liquid changes to a gas. A substantial portion of the carbon dioxide then leaves the well and forces or carries out with it an appreciable amount of the oil or aqueous vehicle utilized to transport the proppant.
U.S. Pat. No. 3,368,627 discloses a method of treating a formation penetrated by a well bore which consists essentially of injecting down the well bore a fluid azeotropic mixture which has a critical temperature sufficiently high or a critical pressure sufficiently low to remain a liquid at the temperature and pressure existing during injection and treatment of the formation. The fluid mixture has critical properties such that a substantial portion of the injected fluid is converted to a gas upon a release of the pressure applied to the liquid during injection into the formation. The fluid mixture consists essentially of carbon dioxide and at least one C.sub.2 to C.sub.6 hydrocarbon.
U.S. Pat. No. 3,664,422 discloses a method of treating a subsurface earth formation penetrated by a well bore comprising injection of a liquefied gas together with a gelled alcohol into the formation at a pressure sufficient to fracture the formation. The liquefied gas is returned from the formation by vaporization following pressure reduction on the well bore. The gelled alcohol is removed by vaporization during subsequent production from the well leaving only the broken gelling agent in the formation.
It would be desirable to provide a method by which a viscous fluid can be created which includes carbon dioxide to facilitate fluid recovery and which is stable over a broad temperature range and particularly elevated temperatures and is capable of carrying high concentrations of proppant into a subterranean formation.